1. Field of the Invention
The present invention relates to an optical waveguide, in particular, a flexible polymer optical waveguide, and also relates to a process for producing the polymer optical waveguide at low cost.
2. Description of the Related Art
Various kinds of processes for producing a polymer optical waveguide have been proposed, including: (1) a process in which films are impregnated with a monomer, a core portion is selectively exposed to light to alter a refractive index and then the films are stuck together (a selective polymerization process), (2) a process in which a core layer and a cladding layer are formed by coating, followed by application of reactive ion etching to form a clad portion (RIE method), (3) a process in which an ultraviolet ray-curable resin as a polymer material added with a photosensitive material thereto is exposed and developed using a photolithographic technique (a direct exposure method), (4) a process using injection molding, (5) a process in which a core layer and a cladding layer are formed by coating, followed by exposure of the core portion to light to thereby alter a refractive index thereof (a photobleaching process) and others.
Problems to be solved still remain, however, in the above respective processes, as follows: The selective polymerization process (1) is problematic in lamination of the films, the processes (2) and (3) are costly since a photolithographic technique is adopted, the process (4) has a problem in precision of an obtained core diameter and the process (5) has a problem of providing no sufficient difference in refractive index between the core and cladding layers.
While processes excellent in performance and practical in use are currently limited to the processes (2) and (3), each has a cost problem to be solved. Furthermore, neither of the methods (1) and (5) is suitable for formation of a polymer optical waveguide in a flexible plastic substrate with a large area.
As a process for producing a polymer optical waveguide, a process is known in which a patterned substrate (cladding layer) in which a pattern of grooves functioning as capillaries is formed is filled with a polymer precursor material for a core, the polymer precursor material is thereafter cured to form a core layer, and then, a planar substrate (cladding layer) are stuck thereon. According to this process, however, not only the capillary grooves but also the entirety of a clearance between the patterned substrate and the planar substrate are filled with the polymer precursor material, and the polymer precursor material is cured thereby forming a thin layer having the same composition as the core layer. As a result a problem arises in that light is leaked out through the thin layer.
As one method of solving this problem, David Heard has proposed a process for producing a polymer optical waveguide, which comprises joining a pattern substrate on which a pattern of grooves to be capillaries is formed and a planar plate with a clamping jig, and evacuating them to fill capillaries with a monomer solution (for example, see Specification of Japanese Patent No. 3151364).
However, in this process, unless a planar plate is joined using a clamp, other areas are undesirably immersed in monomer solution as well as the core part, hence a precise waveguide structure cannot be formed, being a troublesome process. In addition, this process also has a defect that, when a monomer solution is polymerized to obtain a polymer (solidification), volumetric change occurs, which changes the shape of a core. In addition, the process also has a defect that a polymer obtained by polymerizing a monomer solution and capillaries are partially adhered at removal of capillaries and, therefore, a core shape is destructed.
Furthermore, recently, George M. Whitesides and his colleagues of Harvard University advocate a technique called capillary micromolding as a new technique fabricating a nonostructure and as one example of a soft lithography. This is a process in which a master substrate is fabricated using a photolithographic technique, a nanostructure on the master substrate is imprinted on a polydimethyl siloxane (PDMS) mold using adhesiveness and good peelability of PDMS and a liquid polymer is caused to flow into the mold with the help of capillarity and then hardened. A detailed commentary on the process is described in Scientific American, September, 2001 issue (Nikkei Science (Japanese Periodical) December, 2001 issue).
The technique was filed as a patent application concerning a capillary micromolding method by Kim Enoch et al.: members of a group led by George M. Whitesides of Harvard University (see Specification of U.S. Pat. No. 6,355,198).
Even if a process described in the U.S. patent is applied to the producing of a polymer optical waveguide, it takes a long time to form a core portion, disabling application to mass production because of a small area of the core portion of an optical waveguide. Furthermore, in the course of polymerization of a monomer solution to form a polymer, a change in volume occurs and a shape of the core is deformed, leading to a fault of a large transmission loss (waveguiding loss).